The invention in general relates to nuclear magnetic resonance (NMR), and in particular to radio-frequency (RF) coils and coil assemblies for NMR.
Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil can resonate at the Larmor frequency of a nucleus of interest present in the sample. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in test tubes or flow cells. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y or xcex8-plane. In the following discussion, the term xe2x80x9clongitudinalxe2x80x9d will be normally used to refer to the z-direction, while the term xe2x80x9ctransversexe2x80x9d will be used to refer to the xcex8-direction.
Conventional RF coils used for NMR include helical coils, saddle coils, resonant cavities, and birdcage resonators. For information on birdcage resonators see for example U.S. Pat. Nos. 4,692,705, 6,236,206, and 6,285,189. For information on saddle-shaped and other coils see for example U.S. Pat. Nos. 4,398,149, 4,388,601, 4,517,516, 4,641,098, 4,840,700, 5,192,911, 5,818,232, and 6,201,392.
The measurement sensitivity that can be achieved with an NMR coil increases with the coil quality factor Q and its filling factor n. The quality factor Q can be maximized by reducing coil losses. The filling factor n can be increased by reducing the coil size relative to the sample. At the same time, reducing the coil size relative to the sample can increase magnetic field inhomegeneities. Inhomogeneities in the RF magnetic field adversely affect the measurement sensitivity. Moreover, the coil design and dimensions are constrained by the requirement that the coil resonate in a desired frequency range. The resonant frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0.
The conducting parts of conventional coils are typically formed by wires having a round cross-section, or by conductive patterns formed from a thin sheet. Coils formed from sheets can have relatively high distributed capacitances, and thus relatively low inductances, for given coil sizes and resonant frequencies. The measurement sensitivities achievable with typical sheet-conductor coils can be limited by coil losses.
An NMR probe can include multiple NMR coils, each tuned for performing NMR measurements on a different nucleus of interest. For example, an NMR probe can include one coil for performing NMR measurements on protons, and another coil for performing NMR measurements on other nuclei of interest, such as 13C or 15N. In such an NMR probe, the design of one coil can affect the performance of the other coil(s). In order to reduce the coupling between two coils, the coils can be disposed in a quadrature configuration, so that the magnetic fields generated by the coils are mutually orthogonal. This configuration minimizes the mutual inductance between the coils.
The uniformity of the static and RF magnetic fields can be improved by employing susceptibility-compensated wires or sheets for the coil conductors. Abrupt changes in magnetic susceptibility near the sample can significantly degrade the field uniformity in the sample region. A susceptibility-compensated coil has an effective magnetic susceptibility roughly equal to that of the coil environment, which is commonly air or vacuum. A susceptibility-compensated sheet coil can be formed from a sandwich of two or more layers, wherein the materials and thicknesses of the layers are chosen to generate a desired effective susceptibility for the coil. For example, a paramagnetic layer and a diamagnetic layer can be stacked together to yield an effective susceptibility much lower in absolute value than the susceptibility of either layer alone. Effectively achieving susceptibility compensation can constrain RF coil thicknesses. For example, a relatively thin sheet may be required for achieving an effective coil susceptibility approaching zero. For further information on susceptibility-compensated NMR coils see for example U.S. Pat. Nos. 3,091,732 and 6,054,855.
As is apparent from the above discussion, the design of RF coils for NMR applications is highly constrained by multiple parameters. Maximizing the measurement sensitivity of an RF coil or coil assembly typically involves a balancing of effects of various design parameters. Coil designs providing increased design flexibility to the system designer would allow building NMR systems with increased sensitivities.
The present invention provides radio-frequency coils and coil assemblies allowing reduced RF losses, improved NMR measurement sensitivities, and improved magnetic field homogeneities. A radio-frequency coil for an NMR probe and spectrometer includes a plurality of rounded-edge longitudinal conductors interconnecting a pair of longitudinally-spaced, generally transverse ring-shaped sheet conductors. In the presently preferred embodiment, each rounded-edge longitudinal conductors is formed by a generally-longitudinal curled sheet including a tubular or cylindrical portion. The curled sheet and the transverse ring-shaped conductors can be formed from a single susceptibility-compensated sheet. In another embodiment, each longitudinal conductor is formed by a tubular solid wire. The RF coil can be a birdcage coil or a saddle-shaped coil. The rounded-edge sheet coil can be provided as part of a coil assembly including an additional radio-frequency coil disposed orthogonally and concentrically with respect to the rounded-edge coil.
The transverse sheet conductors provide for relatively large distributed capacitances, while the rounded longitudinal conductors allow reducing coil losses and provide for increased transparency to orthogonal magnetic fields. Rounding the coil edges around the coil windows allows reducing the concentration of RF current at the coil edges, and thus reducing the RF losses within the coil. The reduced RF losses allow improved measurement sensitivities. The rounded shapes of the longitudinal conductors also allows increasing the transparency of the longitudinal conductors to the magnetic field generated by the additional, orthogonal coil.
A sheet-curling tool comprising a rotatable, slotted longitudinal curling member simplifies the manufacture of a rounded-conductor coil from a single sheet. The curling member has a longitudinal slot extending along the member, for retaining a sheet conductor to be curled. The slot terminates in a distal open end at a longitudinal end of the curling member. The distal open end allows inserting and removing the sheet conductor longitudinally into/from the curling member. The curling member is mounted on a support comprising two longitudinally-spaced, generally transverse parallel plates. A pair of longitudinal guides is mounted on one of the plates. The longitudinal guides pass through corresponding apertures in the other plate, and allow the two plates to slide longitudinally to grip a patterned sheet in place between the plates. Once the sheet is secured, a wing of the patterned sheet is inserted into the longitudinal slot of the curling member, and the curling member is rotated about its longitudinal axis to curl the wing into a cylinder.